The invention concerns a radio frequency (RF) resonator system, in particular, for an NMR (nuclear magnetic resonance) probe head, comprising at least one RF resonator which has at least three continuous conducting elements which form at least one window in the RF resonator, wherein only part of the conducting elements delimits the at least one window, wherein the conducting elements are coupled to each other via electromagnetic fields, and each conducting element has a limiting current density which is caused by the material, is position-dependent along the conducting element, and must not be exceeded during operation.
Radio frequency resonator systems of this type are disclosed. e.g. in U.S. Pat. No. 5,276,398 (corresponding to reference literature [8]).
Nuclear magnetic resonance (NMR) methods are used to analyse sample compositions or to determine the structure of substances in samples. In these NMR methods, the sample is exposed to a strong static magnetic field. Transverse radio-frequency electromagnetic pulses are irradiated into the sample causing interaction with the nuclear spins of the sample material. The time dependence of these sample nuclear spins generates radio-frequency electromagnetic fields which are detected in the NMR apparatus. The detected radio frequency (RF) fields provide information about the properties of the sample.
RF resonator systems are used to irradiate RF pulses into the sample and also to detect radio-frequency electromagnetic fields from the sample. An RF resonator system comprises one or more RF resonators. The at least one RF resonator is disposed in the direct vicinity of the sample for NMR measurement. The RF resonator system is galvanically, capacitively or inductively coupled to a transmitting or receiving electronics in the NMR apparatus.
Each RF resonator typically comprises several continuous conducting elements which are electromagnetically coupled, i.e. a current flow in one of the conducting elements is accompanied by a current flow in the other conducting elements via electromagnetic coupling. All conducting elements together represent an RF resonant circuit whose resonant frequency is adapted to the NMR experiment (in particular the static magnetic field strength).
To be able to detect NMR signals with minimum noise, NMR ressonators elements are conventionally produced from high-temperature superconducting (HTS) material. Such RF resonators are described in documents [1] to [10]. One disadvantage of these HTS resonators is that the maximum manageable transverse RF magnetic field strength (B1 field strength) of these RF resonators is limited by the critical current density jc of the HTS material. The larger the manageable RF magnetic field strength in the RF resonators, the shorter may be the RF pulses used by the NMR apparatus and the greater the excitation bandwidth.
It is therefore the underlying purpose of the present invention to present RF resonator systems which permit maximum transverse RF magnetic field strengths and also have a largely quadratic dependence between irradiated power and magnetic field increase in the RF resonator, i.e. the resonator is operated in a linear region of the resistance characteristic.